THE ZONA REACTION IN HUMAN OOCYTES AS SEEN WITH SCANNING ELECTRON-MICROSCOPY

Using scanning electron microscopy we found differences in the fine st ructure of the zona pellucida between unfertilized and fertilized huma n pronuclear stage oocytes in an in-vitro fertilization programme. In unfertilized oocytes, the zona pellucida appeared porous, comprising a large number of ring-shaped structures, called hoops, randomly superi mposed in several layers. Superficial pores had a mean diameter of 4 m u m, with the diameter decreasing in more inner lying pores. In fertil ized oocytes, the zona pellucida was compact; the hoops appeared to me lt and the pores to be obliterated by an amorphous material emerging f rom the inner zona. The micrographs provide ultrastructural evidence o f the zona reaction in human oocytes and give insights into the morpho logical and mechanical aspects of the polyspermy-blocking mechanism in humans.